BACILLACE^: THE SPOROGENIC ANAEROBES 



293 



many times greater than the amount necessary to kill a normal 

 animal. Moreover, the cell-free blood serum of the immunized 

 animal was found to neutralize the poison in a test-tube and to 

 protect a normal animal against fatal 

 doses of it. The new substance of the 

 blood capable of rendering the toxin 

 harmless was -called antitoxin. One 

 antitoxic unit of tetanus antitoxin, ac- 

 cording to Von Behring is the amount 

 which will neutraUze 40 milhon times 

 the amount of fresh tetanus toxin 

 necessary to kill a mouse weighing 1 5 

 grams (40 million X the 15 + Ms 

 dose) so completely that only a slight 

 local contraction, indicated by a fold- 

 ing of the skin, results from sub-v 

 cutaneous injection of the mixture into 

 a mouse (the Lo effect) . This amount 

 of toxdn (40 million X the 15 + Ms 

 dose) is generally measured in practice 

 against a standard antitoxin and is 

 designated as a toxic unit. The toxin 

 is preserved in a dry state. To test a 



new antitoxin one employs of a 



^ ■" 1000 



toxic unit (40,000 X the 15 + Ms 



dose) and ascertains the amount of 



serum which must be added so as to 



neutralize it to the Lo end point. 



diluted to i c.c. with salt solution and 0.25 c.c. per 10 grams of 



body weight is injected into a mouse. When the typical Lo effect 



is produced in the mouse, the amount of antitoxic serum employed 



in the preparation of this particular mixture is said to represent 



antitoxic unit. Ordinarily the mixture of toxin and anti- 



1000 



F IG. II 7. — Clostridium letani. 

 Stab culture in glucose gelatin, 

 six days old. (From McFarland 

 after Fraenkel and Pfeiffer.) 



Each trial mixture is 



